Video: Triggering lights with guitar frequency levels

Had an idea after catching this post on Hack a Day, why not use the frequency level values to trip a 120 volt relay? So I ordered some parts and did it. The audio analyzing chip, the MSGEQ7, is easily accessed using DFRobot’s DFR0126, which, being in Canada, I got from RobotShop. Connecting the breakout board to an Arduino Nano was a 5 minute job, sample code and a library is linked from the DFRobot product page. I initially used a potentiometer to input the threshold levels for the relay, but then realized I could use a momentary switch to sample the desired threshold and then use that to compare the real-time input to.

The circuit is simple, when a button (momentary stomp) is depressed, and we all get depressed sometimes, the code saves the input values from the audio analyzer. There are seven frequency bands it records, but I found only three or four of them are applicable to guitar, so ignore the lowest and perhaps the highest two. After a threshold has been recorded simply check the input against the recorded levels and trip the relay (or not).

I gave the thresholds a grace of 5 (on a theoretical input range of 0-1023), I may add a pot for this adjustment as it may vary based on guitar signal types. The result is quite versatile, you could have the relay turn off a mellow light and turn on a spastic light when the signal goes loud. If you pay close enough attention to EQ bands and levels you could trigger various lights based on a variety of guitar effects. This setup would also allow, albeit in a roundabout way, you to engage a guitar effect based on the frequency band levels, as long as the effect will pass-through without power then connecting it to the relay would engage the effect — or you could redesign this circuit to route some audio signals based on the input levels.

The pedal I stomp in the video is the MP-1 fuzz from Inductor Guitars, the EQTrigger pedal is connected to the extra output on a Boss TU-2 tuner and is reacting auto-magically to the change in guitar signal when I play louder or engage the fuzz.